Fast frequency synthesizer concept based on digital tuning and I/Q signal processing

The design of frequency synthesizers is especially challenging for wireless applications due to the requirements for high spectral purity, high frequency range, and fast tuning together with reasonable power consumption. The idea of combining digital and analog synthesis techniques for achieving these goals is discussed and analyzed. The proposed architecture uses I/Q modulation to translate a digitally synthesized tuneable low frequency tone to the final frequency range. In practical implementations, however, unavoidable mismatches between the amplitudes and phases of the I and Q branches result in imperfect sideband rejection degrading the spectral purity of the synthesized signal. A compensation structure based on digital pre-distortion of the low frequency tone is presented to enhance the signal quality. Furthermore, practical algorithms for updating the compensator parameters are proposed based on minimizing the envelope variation of the synthesizer output signal. Simulation results are also presented to illustrate the efficiency of the proposed synthesizer concept.